NASA's Kepler Space Telescope has discovered more than two thousand exoplanets since launching seven years ago, and a newly compiled catalogue of candidates could soon give this tally a sizeable boost. But it's not all about the quantity for this orbiting telescope, with the data suggesting that the exoplanets we've discovered can be placed into two distinct categories, helping scientists determine the demographics of planets that make up our galaxy.
In total, Kepler has identified 4,034 exoplanet candidates, 2,335 of which have been verified. This process involves following up the initial detection with observations, and requires researchers to verify candidates one-by-one, although new methods are making things easier. More than 30 of those candidates have been found to be Earth-sized and orbit in the habitable zone – just the right distance from their star for liquid water to potentially form on the surface.
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The latest set of candidates comes from scientists who reprocessed the data from Kepler's first four years of its primary mission, as it observed a patch of sky in the Cygnus constellation. It reveals another 219 exoplanet candidates, with 10 of those close to the size of Earth and falling in their star's habitable zone.
And this data has revealed an interesting trend as part of a second study, with scientists saying that most of the planets discovered by Kepler so far fall into two separate size classes. The first is a group of rocky, Earth and super-Earth-sized planets such as Kepler-425b, which NASA described as Earth's bigger, older cousin when it was discovered in 2015. The other group is made up of gaseous planets smaller than Neptune, like Kepler-22b which was discovered in 2011.
"We like to think of this study as classifying planets in the same way that biologists identify new species of animals," said Benjamin Fulton, doctoral candidate at the University of Hawaii in Manoa, and lead author of the second study. "Finding two distinct groups of exoplanets is like discovering mammals and lizards make up distinct branches of a family tree."
The video below provides an overview of the discovery.